Effects of Land Use Distribution on the Surface Water Quality in the Upstream Section of Nairobi River

Abstract

The river ecosystems play critical roles in ensuring the general human wellbeing and social-economic development. Despite their importance, surface water resources and rivers in particular face threats from several factors and key among them is the changes in land use in a watershed. Pollution of rivers contributes to the growing problem of water stress around the World, Kenya is not an exception to this experience. The main objective of this study was to understand the effects of spatial land use distribution on water quality characteristics in in the upstream section of Nairobi River extending from the source at Ondiri Swamp up to its cross section at Naivasha Road Bridge, a stretch of 10.2 kilometers. A riparian buffer strip of 500 meters on either side of the river bank was adopted for the study to allow data collection of water samples and land use types. The main objective of the study was achieved by answering the questions of what the land use types were in the study area, the state of physical, chemical, and biological water quality characteristics, and what relationship existed between the land use types and the water quality parameters. The study used GIS and remote sensing to analyze the land use types in the study area using ArcGIS version 10.8. Water quality was collected by applying systematic random sampling at an interval range of 2.0 km. and ensuring that each point was located at the tributary confluence and exhibiting distinct land use type. Water samples were analyzed in situ and at the University of Nairobi Public Health Engineering laboratory. Transportation of samples was done using marked glass bottles and a cooler box to avoid degeneration of samples. All samples were tested within 24hrs of collection. The relationship between the land use type and water quality was analyzed using the nonparametric Spearman’s correlation test using R program version 4.0.2. The study results showed that the main land use types were agricultural, urban-settlement mix, and wetland. The water quality parameters showed significant differences between the dry and wet seasons as well as sampling sites along the river gradient. In both seasons, water quality was noted to deteriorate downstream with no discernible trends partly due to random effluence discharge at various points of the river and the hydro-geochemical processes within the river system. The wetland was found to influence water quality parameters that are indicative of pollution positively, while the urban settlement land use was found to degrade water quality particularly in the wet season. For instance, Spearman’s analyses showed that pH was strongly negatively correlated with wetland in both seasons but had a strong positive correlation with residential land use, these correlations were significant at p<0.05. There was however strong and significant positive correlation between settlement areas and Total faecal coliforms at 0.954 with a computed P value of 0.01 (at P<0.05). These residential areas did not have adequate sewerage infrastructure and their location within the 500m riparian area, limited vegetation cover exacerbated the effluents drainage to the river. The study concluded that there was evidence of effects of land uses on water quality based on the degree of anthropogenic modification on a particular land use. The less modified wetland had less impact on water quality by filtering the pollutants though the encroachment could degrade the riparian vegetation around the wetland. The research recommends that increased residential densities be matched with commensurate water and sewer management infrastructure to manage levels of fecal coliform elements into the river system. Increment of vegetation cover to act as buffer zone along the riparian land and use of storm retention dam with complementary treatment works were recommended to the south of ILRI. At a policy level, the river water is expected to be suitable for augmenting water supply to Nairobi City and adjoining Counties.